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<div class="titlepage"><div><div><h3 class="title">
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool"></a><a class="link" href="BoyerMooreHorspool.html" title="Boyer-Moore-Horspool Search">Boyer-Moore-Horspool
      Search</a>
</h3></div></div></div>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h0"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.overview"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.overview">Overview</a>
      </h5>
<p>
        The header file 'boyer_moore_horspool.hpp' contains an implementation of
        the Boyer-Moore-Horspool algorithm for searching sequences of values.
      </p>
<p>
        The Boyer-Moore-Horspool search algorithm was published by Nigel Horspool
        in 1980. It is a refinement of the Boyer-Moore algorithm that trades space
        for time. It uses less space for internal tables than Boyer-Moore, and has
        poorer worst-case performance.
      </p>
<p>
        The Boyer-Moore-Horspool algorithm cannot be used with comparison predicates
        like <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">search</span></code>.
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h1"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.interface"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.interface">Interface</a>
      </h5>
<p>
        Nomenclature: I refer to the sequence being searched for as the "pattern",
        and the sequence being searched in as the "corpus".
      </p>
<p>
        For flexibility, the Boyer-Moore-Horspool algorithm has two interfaces; an
        object-based interface and a procedural one. The object-based interface builds
        the tables in the constructor, and uses operator () to perform the search.
        The procedural interface builds the table and does the search all in one
        step. If you are going to be searching for the same pattern in multiple corpora,
        then you should use the object interface, and only build the tables once.
      </p>
<p>
        Here is the object interface:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">patIter</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">boyer_moore_horspool</span> <span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">boyer_moore_horspool</span> <span class="special">(</span> <span class="identifier">patIter</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">patIter</span> <span class="identifier">last</span> <span class="special">);</span>
    <span class="special">~</span><span class="identifier">boyer_moore_horspool</span> <span class="special">();</span>

    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">corpusIter</span><span class="special">&gt;</span>
    <span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">corpusIter</span><span class="special">,</span> <span class="identifier">corpusIter</span><span class="special">&gt;</span> <span class="keyword">operator</span> <span class="special">()</span> <span class="special">(</span> <span class="identifier">corpusIter</span> <span class="identifier">corpus_first</span><span class="special">,</span> <span class="identifier">corpusIter</span> <span class="identifier">corpus_last</span> <span class="special">);</span>
    <span class="special">};</span>
</pre>
<p>
      </p>
<p>
        and here is the corresponding procedural interface:
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">patIter</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">corpusIter</span><span class="special">&gt;</span>
<span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">corpusIter</span><span class="special">,</span> <span class="identifier">corpusIter</span><span class="special">&gt;</span> <span class="identifier">boyer_moore_horspool_search</span> <span class="special">(</span>
        <span class="identifier">corpusIter</span> <span class="identifier">corpus_first</span><span class="special">,</span> <span class="identifier">corpusIter</span> <span class="identifier">corpus_last</span><span class="special">,</span>
        <span class="identifier">patIter</span> <span class="identifier">pat_first</span><span class="special">,</span> <span class="identifier">patIter</span> <span class="identifier">pat_last</span> <span class="special">);</span>
</pre>
<p>
      </p>
<p>
        Each of the functions is passed two pairs of iterators. The first two define
        the corpus and the second two define the pattern. Note that the two pairs
        need not be of the same type, but they do need to "point" at the
        same type. In other words, <code class="computeroutput"><span class="identifier">patIter</span><span class="special">::</span><span class="identifier">value_type</span></code>
        and <code class="computeroutput"><span class="identifier">curpusIter</span><span class="special">::</span><span class="identifier">value_type</span></code> need to be the same type.
      </p>
<p>
        The return value of the function is a pair of iterators pointing to the position
        of the pattern in the corpus. If the pattern is empty, it returns at empty
        range at the start of the corpus (<code class="computeroutput"><span class="identifier">corpus_first</span></code>,
        <code class="computeroutput"><span class="identifier">corpus_first</span></code>). If the pattern
        is not found, it returns at empty range at the end of the corpus (<code class="computeroutput"><span class="identifier">corpus_last</span></code>, <code class="computeroutput"><span class="identifier">corpus_last</span></code>).
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h2"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.compatibility_note"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.compatibility_note">Compatibility
        Note</a>
      </h5>
<p>
        Earlier versions of this searcher returned only a single iterator. As explained
        in <a href="https://cplusplusmusings.wordpress.com/2016/02/01/sometimes-you-get-things-wrong/" target="_top">https://cplusplusmusings.wordpress.com/2016/02/01/sometimes-you-get-things-wrong/</a>,
        this was a suboptimal interface choice, and has been changed, starting in
        the 1.62.0 release. Old code that is expecting a single iterator return value
        can be updated by replacing the return value of the searcher's <code class="computeroutput"><span class="keyword">operator</span> <span class="special">()</span></code>
        with the <code class="computeroutput"><span class="special">.</span><span class="identifier">first</span></code>
        field of the pair.
      </p>
<p>
        Instead of:
</p>
<pre class="programlisting"><span class="identifier">iterator</span> <span class="identifier">foo</span> <span class="special">=</span> <span class="identifier">searcher</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">);</span>
</pre>
<p>
      </p>
<p>
        you now write:
</p>
<pre class="programlisting"><span class="identifier">iterator</span> <span class="identifier">foo</span> <span class="special">=</span> <span class="identifier">searcher</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
</pre>
<p>
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h3"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.performance"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.performance">Performance</a>
      </h5>
<p>
        The execution time of the Boyer-Moore-Horspool algorithm is linear in the
        size of the string being searched; it can have a significantly lower constant
        factor than many other search algorithms: it doesn't need to check every
        character of the string to be searched, but rather skips over some of them.
        Generally the algorithm gets faster as the pattern being searched for becomes
        longer. Its efficiency derives from the fact that with each unsuccessful
        attempt to find a match between the search string and the text it is searching,
        it uses the information gained from that attempt to rule out as many positions
        of the text as possible where the string cannot match.
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h4"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.memory_use"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.memory_use">Memory
        Use</a>
      </h5>
<p>
        The algorithm an internal table that has one entry for each member of the
        "alphabet" in the pattern. For (8-bit) character types, this table
        contains 256 entries.
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h5"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.complexity"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.complexity">Complexity</a>
      </h5>
<p>
        The worst-case performance is <span class="emphasis"><em>O(m x n)</em></span>, where <span class="emphasis"><em>m</em></span>
        is the length of the pattern and <span class="emphasis"><em>n</em></span> is the length of
        the corpus. The average time is <span class="emphasis"><em>O(n)</em></span>. The best case
        performance is sub-linear, and is, in fact, identical to Boyer-Moore, but
        the initialization is quicker and the internal loop is simpler than Boyer-Moore.
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h6"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.exception_safety"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.exception_safety">Exception
        Safety</a>
      </h5>
<p>
        Both the object-oriented and procedural versions of the Boyer-Moore-Horspool
        algorithm take their parameters by value and do not use any information other
        than what is passed in. Therefore, both interfaces provide the strong exception
        guarantee.
      </p>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h7"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.notes"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.notes">Notes</a>
      </h5>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            When using the object-based interface, the pattern must remain unchanged
            for during the searches; i.e, from the time the object is constructed
            until the final call to operator () returns.
          </li>
<li class="listitem">
            The Boyer-Moore-Horspool algorithm requires random-access iterators for
            both the pattern and the corpus.
          </li>
</ul></div>
<h5>
<a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.h8"></a>
        <span class="phrase"><a name="the_boost_algorithm_library.Searching.BoyerMooreHorspool.customization_points"></a></span><a class="link" href="BoyerMooreHorspool.html#the_boost_algorithm_library.Searching.BoyerMooreHorspool.customization_points">Customization
        points</a>
      </h5>
<p>
        The Boyer-Moore-Horspool object takes a traits template parameter which enables
        the caller to customize how the precomputed table is stored. This table,
        called the skip table, contains (logically) one entry for every possible
        value that the pattern can contain. When searching 8-bit character data,
        this table contains 256 elements. The traits class defines the table to be
        used.
      </p>
<p>
        The default traits class uses a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">array</span></code>
        for small 'alphabets' and a <code class="computeroutput"><span class="identifier">tr1</span><span class="special">::</span><span class="identifier">unordered_map</span></code>
        for larger ones. The array-based skip table gives excellent performance,
        but could be prohibitively large when the 'alphabet' of elements to be searched
        grows. The unordered_map based version only grows as the number of unique
        elements in the pattern, but makes many more heap allocations, and gives
        slower lookup performance.
      </p>
<p>
        To use a different skip table, you should define your own skip table object
        and your own traits class, and use them to instantiate the Boyer-Moore-Horspool
        object. The interface to these objects is described TBD.
      </p>
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<td align="right"><div class="copyright-footer">Copyright © 2010-2012 Marshall Clow<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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